It's not cheap but in my opinion the Singh ray 8 stop vari nd filter is worth every penny.
It will cover the full range of stops without compromising image quality.
I use mine all the time... And couldnt live without it.
I totally agree ... but unfortunately, I don't use it as often as I should
... I've got too many do dads and I get distracted with them and forget to carry some of the important filters
5D MK III, 16-35/2.8 L II, 24-70/2.8 VC, 24-105/4 L IS, 70-200/2.8 L IS II, 100/2.8 L IS, 40/2.8, 50/1.4, 85/1.8, 150-500 OS, 600EX-RTx2, ST-E3-RT, Rokinon 24/3.5 T-S, D7100+18-300 VR, YN568EX, 055CXPRO4+468MGRC2, 190CXPRO4+322RC2, Gorillapod Focus+496RC2, CamRanger, B+W filters, Konova K3/31.5 etc